The present invention relates to a switching power supply circuit for a direct connection to an AC power outlet. More generally it relates to a frequency modulator circuit which is suited for modulating the switching frequency of a power supply to produce a rectified AC voltage.
The so-called switching type power supplies are widely used circuits within electronic and electromechanical appliances. They provide a low-voltage DC supply through a rectifier circuit which may be connected to the AC power outlet without using a step-down transformer. However, the absence of a transformer permits electromagnetic disturbances to propagate into the power network. This is undesirable, and is likely to be increasingly constrained by regulations.
The main frequency of these disturbances is related to the switching frequency of the power supply circuit. This frequency may correspond to a value which interferes with telecommunication circuits using the power distribution line as a signal path.
Commonly the oscillator used for determining an oscillation frequency for a switching-type power supply contains a ramp generator circuit. In this circuit, an output ramp signal is fed to an input of a comparator. Another input of the comparator is fed with a reference voltage to determine a triggering threshold. This charges and discharges capacitors of the ramp generator. The resulting frequency of oscillation is predetermined by selecting the appropriate resistance and capacitance values (i.e. by selecting an RC time constant). The ramp generator may contain a constant current generator which itself may contain a buffer capable of forcing a certain constant current through a resistance. The constant current is mirrored in the charge/discharge network of the capacitor of the ramp generator circuit.
In the present invention it has been found that by modulating the oscillation frequency to have an instantaneous frequency value which is inversely proportional to the instantaneous amplitude of the rectified AC voltage two important effects can be achieved. First, by reducing the oscillation frequency under conditions of high current, the electromagnetic energy associated with disturbances is reduced. Secondly, a reduction of the oscillation frequency when current is high also reduces power loss through the power switch. In other words, under conditions of high current, a reduction in frequency results in a more efficient and noise free circuit.
In a preferred embodiment of the invention, the modulation of the oscillation frequency in a rectifier circuit containing a voltage ramp generator is implemented by means of a divider having two inputs and an output. The first input fed with the output of a standard bridge rectifier. The second input is fed with the RMS value of the rectified voltage. This voltage may be obtained as the output of a low-pass filter which may be connected across the output nodes of the bridge rectifier.
The voltage divider circuit generates an output current which is proportional to the ratio of its two inputs (proportional to Vin, and inversely proportional to Vrms). The output current (Iout) generated by the divider is used to modulate the oscillation frequency and has an instantaneous value of the rectified AC voltage (Vin). Therefore, the value of the output current (Iout) is dependent on only the design parameters of the circuit itself and not the value of the RMS-AC voltage. (This voltage may vary in different countries.)
The output node of the frequency modulating circuit (i.e. the divider circuit) may be directly connected to the load used by the constant current source of the ramp generator circuit. This is done in order to subtract a variable current generated by the modulating circuit.